1. Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly to an image pickup apparatus equipped with a diffraction grating optical low-pass filter disposed between a photographic lens and an image pickup element.
2. Description of the Related Art
An image pickup apparatus, such as a video camera or a digital still camera, includes a photographic lens for receiving light incident from an object, an image pickup element for picking up an image of the object, and an optical low-pass filter disposed between the photographic lens and the image pickup element. The image pickup element is implemented by a single plate type image pickup tube or a solid-state image pickup element (CCD), for example.
To cut off high-frequency components contained in light from the object, the optical low-pass filter has a plurality of quartz crystals formed on a surface thereof toward an image forming surface of the image pickup element, such that the quartz crystals are arranged while varying optical axes thereof. This makes it possible to eliminate moire and false colors caused by a light pattern of the high-frequency components adversely affecting pixels of the image pickup element.
An example of the optical low-pass filter described above is one proposed in Japanese Laid-Open Patent Publication (Kokai) No. H01-254912 which includes a diffraction grating (hereinafter referred to as “the diffraction grating optical low-pass filter”) (Pages 3 and 4, FIG. 1 of the publication). This optical low-pass filter includes no quartz crystals, since it includes the diffraction grating. Further, in the diffraction grating optical low-pass filter proposed in Japanese Laid-Open Patent Publication (Kokai) No. H01-254912, quadratic prism-shaped unit cells, which form the diffraction grating for use in the optical low-pass filter, are two-dimensionally arranged.
Further, Japanese Laid-Open Patent Publication (Kokai) No. H01-254912 discloses that intensities of luminance signals corresponding to optical images formed on a surface of the image pickup element are averaged by a pattern of the diffraction grating.
However, the above-described optical low-pass filter that includes quartz crystals has disadvantages in cost and space since the filter is required to have the plurality of quartz crystals arranged while varying optical axes thereof.
On the other hand,
the diffraction grating optical low-pass filter is more inexpensive and advantageous in respect of required space than the optical low-pass filter including the quartz crystals, but it suffers from the problem that when the aperture of the photographic lens is stopped down, there is produced reflections (shadows) of optical images caused by the structural pattern of the diffraction grating. Particularly in an image pickup apparatus of a single-lens reflex type, which has to be configured such that various lenses can be exchanged, it is necessary to suppress occurrence of shadows from being caused even by a light flux formed by an aperture having an F-number of 16 or more, though such a light flux is not required by compact digital cameras.
Further, in the diffraction grating optical low-pass filter, the pattern of an optical image formed on the surface of the image pickup element changes intricately not only due to the structural pattern of the diffraction grating but also due to a gap (spacing) between the diffraction grating and the image pickup element. For example, even under general shooting conditions, an optical image sometimes has portions strong in light intensity, which are locally produced by a change in the pattern of the optical image. Furthermore, the image pickup element is not open to a full opening percentage of 100%, and hence it is difficult to average light amounts in the portions locally strong in light intensity. Therefore, when a light flux formed by an aperture having an F-number of 16 or more is incident, it is difficult to suppress occurrence of shadows generated by the light flux simply by using the diffraction grating.